Lubricant Concepts for Hybrid Electric Vehicle (HEV) Transmission

It is anticipated that many future hybrid electric vehicle (HEV) transmissions will incorporate the e-motor within the transmission housing while being constrained to ever smaller packaging dimensions. The heat from the e-motor confined to the smaller designs will inevitably lead to higher transmission operating temperatures, necessitating the use of higher temperature resistant plastics and requiring the efficient removal of excess heat.

This will pose new challenges to lubricants. In addition to ensuring the lubricant will be durable in the harsher thermal environment, it must also be compatible with these new plastics and able to protect them from thermal degradation products. The ability to transfer heat will become a critical factor in automatic transmission fluid design, especially for e-transmission devices, when such transmissions are hotter. In this paper the heat transfer characteristics of lubricants are reviewed as well as the fundamental factors that impact them and how the drive for greater heat transfer will drive the lubricants to have lower viscosities. This will have a secondary impact on other fluid attributes, such as hardware protection and electrical properties, and the strategies to deal with these are outlined. The lubricant must be compatible with various plastics and this is described also by way of tensile strength testing.